Currently, ultrasonic transducers are typically formed of piezoelectric materials for transmission and reception of interrogating ultrasonic waves transmitted through biologic tissues or materials. The corresponding piezoelectric elements are commonly made from polycrystalline ceramics such as lead-zirconate-titanate or ceramic-polymer composites having ceramic rods embedded in a matrix of resin. The intrinsic advantages of piezoelectric transducers are well known in the art and include such advantages as high energy conversion factors and suitability for low volume production. Unfortunately, the shortcomings of this technology are numerous as well, and the various disadvantages include a low reproducibility of the piezoelectric characteristics, aging and temperature sensitivity, and a lack of suitability for mass production or complex miniaturization.
Since the 1960s, other forms of ultrasonic transducers have been developed and disclosed in the prior art which use an electrostatic force for moving capacitive membranes. The basic principle is quite simple and has been successfully implemented in condenser microphones having passive components. For capacitive transducers, the operation is governed by a voltage oscillation over its electrostatic field. This oscillation causes the membrane to vibrate, therefore producing the emission of ultrasonic waves. Conversely, the reception of a pressure force at the surface of biased membranes will cause deformation of the surface thereby resulting in oscillation of the output voltage. Unlike piezoelectric transducers that perform very well with solid interfaces, capacitive membrane transducers are more suitable in air and liquid based applications. The capacitive membranes are commonly microfabricated on a silicon substrate using etching technologies used for CMOS circuits.
One such transducer is called a Capacitive Micromachined Ultrasonic Transducer (CMUT). CMUT devices can be obtained using well known semiconductor manufacturing processes similar to those employed in CMOS or Bi-CMOS technologies.
Considering these devices in more detail, the diameter and thickness of the membranes are defined according to desired characteristics of the transducer. In most cases, the CMUT cells are preferably microfabricated on a suitable material substrate such as silicon (Si). Because the diameter of CMUT cells are governed by the operating frequency of the transducer, the sizes range from a few microns to dozens of microns. Therefore, to form the complete surface of the transducer, hundreds or thousands of cells must then be electrically connected in parallel. The transducer so obtained can also easily be combined with electric impedance matching circuitry or control circuitry to form an integrated transducer assembly ready to be housed or cable connected. The packaging used is defined or determined upon request according to the particular applications or customer specifications.
The manufacture of CMUT cells for immersion transducers has been disclosed in the prior art. For example, U.S. Pat. No. 5,894,452 to Ladabaum et al discloses cells formed from a highly doped silicon substrate having membrane supports of silicon dioxide and sealed membranes of silicon nitride.
U.S. Pat. No. 5,619,476 to Haller et al. discloses an electrostatic ultrasonic transducer in combination with a manufacturing method which seeks to avoid collapsing of the nitride membrane during the etching process. Membranes of circular and rectangular shapes are also described.
In U.S. Pat. No. 5,870,351 to Ladabaum et al., a broadband microfabricated ultrasonic transducer is disclosed wherein a plurality of resonant membranes of different sizes are provided. Each size of membrane is responsible for a predetermined frequency so an extended bandwidth for the transducer can be expected. Further, the membranes may be made in various forms and shapes.
Another aspect of membrane fabrication is taught in U.S. Pat. No. 5,982,709 to Ladabaum et al, wherein polysilicon or silicon nitride membranes are deposited on a support structure specially tailored to minimize the effect on the vibration of the membranes. Typically, etching holes are formed in the area external to the membranes so as to not disturb the operation thereof.
WO 02091796A1 to Foglietti et al discloses the use of silicon monoxide as support material for membranes. In one embodiment, a chromium sacrificial material is employed and, alternatively, an organic polymer (polyamide) may be used. The chemical etching of chromium or polyamide is more selectively controlled as compared with silicon dioxide. The polyamide material is spin coated and then dry etched in a manner such as to control the thickness (500 nm.) This, in turn, governs the gap provided between the membrane and the substrate. A PECVD process is used for film growth.
It will be understood that with respect to the above-described prior art, electrostatic cells for ultrasonic transmissions must be designed according to the operating specifications, i.e., center frequency, bandwidth and sensitivity. These specifications are interdependent, i.e., are cross-linked to each other through the design of the cells. In this regard, it is well known that the frequency and bandwidth of transducer are governed by the diameter and thickness of the membranes and, in general, the gap between the membranes/substrate and the thickness of membrane contribute to the control of the collapse voltage and thus to the sensitivity of the cells. Obviously, such factors as the stiffness (Young's modulus) of the membrane and the membrane geometry will also play major roles in the acoustical operations of the cells.
In general, and for operations involving ultrasonic applications, in emission (transmission) operations, the maximum Coulombian force is required on the membrane in order to provide a high displacement amplitude of the membrane. This force should, however, be controlled so as to prevent collapse of the membrane onto the cavity bottom surface. In reception operations, where a pressure force is exerted on the membrane surface, the electrical sensitivity is governed both by the biasing voltage and the capacitance observed between the electrodes. Reduction of the membrane thickness inherently leads to a decrease in the biasing voltage, thereby optimizing the reception voltage measured on the cells.
In the related prior art, no cell or transducer construction has fully taken into account the particularities of the emission and reception of ultrasounds by the electrostatic components discussed above, so there is a need for an electrostatic cell wherein integrated emission and reception functions are provided independently, together with optimization of each particular function and without impacting on the operations of the other.